The micro-photolithography is used in the processing of microelectronic devices. As a pattern with a finer feature size is needed, it is more strictly required to control the physical properties of components of which photoresist compositions are made. For a polymer used as one component of photoresist compositions, for instance, it is crucial to control its molecular weight and dispersity as well as its chemical structure. As used herein, the term “dispersity” representative of a molecular weight distribution is defined as weight average molecular weight divided by number average molecular weight, i.e., Mw/Mn.
It has long been practiced to control the dispersity of a polymer used in photoresist compositions. For example, JP-A 5-142778 discloses a novolak resist composition comprising a novolak resin with a reduced dispersity so that the resist pattern is more heat resistant. JP-A 6-266099 discloses chemically amplified positive and negative resist compositions comprising a vinyl phenolic resin wherein not only the resist technology and the novolak resin are tailored, but also the dispersity of the resin is reduced, thereby improving the roughness of a pattern. JP-A 2002-251009 discloses that a reduction of the dispersity of an acrylic resin having alicyclic structure contributes to a higher resolution.
While low dispersity polymers are obtainable directly through such reaction as living polymerization, polymers synthesized otherwise generally have high dispersity and contain an amount of low-molecular-weight compounds which detracts from resolution. The low-molecular-weight fraction must be removed from such a polymer product by any suitable means of molecular weight fractionation.
The polymer fractionation may generally be achieved by several methods including re-precipitation of polymer, and a liquid phase separation technique of adding a polymer to a solvent system of a good solvent and a poor solvent and selecting a set of conditions under which the good solvent and the poor solvent separate into two distinct phases. Of these, the liquid phase separation technique is advantageous in that this additional step imposes only a small load to the commercial production process because of a high percent recovery and ease of operation in a large scale. For example, U.S. Pat. No. 6,121,412 (or JP-A 2003-529621) discloses molecular weight fractionation of novolak resin by a liquid phase separation technique involving separation of two liquid phases by centrifugation.
The related patent references include JP-A 2004-035671, JP-A 2005-264103, JP-A 2005-344009, JP-A 2006-201532, and JP-A 2006-213835.